Reader method to locate message ID by event time

[candlerb]

Is your feature request related to a problem? Please describe.
I would like to be able to initialize a reader using an event time. Typical use cases:

• I have a long-lived, persistent topic containing system logs, and I want to do an ad-hoc query saying "show me logs starting from 1 hour ago"
• I have a reader which is storing its messageID state in a database, and I want to rewind it to a particular point in time
• I am developing reader code and I want to test it across a subset of recent messages, not from the very beginning of the topic.

Currently when you create a "reader", you can initialize it to start at either the earliest message or the end of the topic - there are special sentinel message IDs for those cases. You can also initialize it to any existing message ID on a topic, but to do that, you must already know a valid message ID.

Describe the solution you'd like
An API call which, given a topic and event time, returns the message ID of the first message after that time. Alternatively, an additional option to createReader which takes an event time[^1]

This functionality must already exist internally in Pulsar because the admin API has a reset-cursor call for subscriptions. I would like that internal search for event time to be exposed.

I suspect this would end up in the REST API rather than the client protocol. Ideally it would be accessible to clients without admin privileges. I found some work to allow subscription-related admin calls to unprivileged consumers when dealing with their own subscriptions - #2964 / #2981 - so I'd like the new call to be covered by this.

Describe alternatives you've considered
For topic X, I could write a separate index topic X1 which emits a messageID and timestamp, say one message for every 10,000 messages in X. Then I can scan X1 looking for the last timestamp before the time of interest, and then read X forward from that messageID. This would have to be duplicated for every topic.

I could create a temporary subscription, use reset-cursor on it, and read using a consumer - or read one message, and use it to get the message ID to initialize a reader. It seems overblown to create and destroy a subscription just for that.

Additional context
Provides feature parity to Kafka: getOffsetsByTimes / offsetsForTimes

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[^1] EDIT: I see that Java Reader API already has seek(timestamp) and seek(messageId) - however Python Consumer has only seek(messageId), and Python Reader is missing seek completely. So I will close this out in favour of #5541

The Java consumer also has subscriptionInitialPosition, but it can only choose between Earliest and Latest.

[candlerb]

Fixed by #5542